This invention relates generally to optical fiber systems for detecting forces such as stress acting upon a structure. More specifically, this invention relates to an optical fiber microbend system and components thereof for inducing microbending of the fiber in response to stress in an oil or gas pipeline or the like and for detecting the microbending of the fiber to provide an indication of the location and magnitude of the stress forces.
Optical fibers in general are known in the art, and typically comprise a transparent core of a suitable glass or plastic material which is carried within a relatively thin cylindrical cladding having an index of refraction less than the refractive index of the core. When a light signal such as a collimated beam generated by a laser is focused upon one end of the fiber, the fiber core functions as a waveguide to transmit or propagate the light signal through the core with relatively small internal intensity losses or transmission of the signal to the cladding. An important feature of optical fibers of this type is that gradual turns or bends in the fiber have little or no effect upon transmission of the light signal, thereby permitting transmission of the light signal through the fiber for emission at the opposite end of the fiber regardless of the number of bends and turns. Practical applications of the such fibers have included, for example, devices to illuminate or to permit viewing of inaccessible areas, such as areas inside the human body, or as a telecommunications link wherein the light signal is modulated to represent information which can be detected and demodulated at the opposite end of the fiber.
It has been recognized, however, that relatively short bends in an optical fiber can have a significant effect upon the transmissivity of the fiber core. More specifically, the presence of a short bend having a period on the order of a few millimeters, commonly referred to as a microbend, results in an attenuation of the propagated light signal which arises by scattering of a portion of the signal from the fiber core to the cladding from where most of the scattered light portion is lost ultimately to the surrounding environment. In some applications, this attenuation phenomenon is a useful characteristic such as when it is desired to tap or extract a portion of the signal from the fiber without cutting the fiber. In this regard, microbend couplers, such as that described in U.S. Pat. No. 4,253,727 have been proposed for inducing one or more microbends into the fiber to extract a portion of the light signal therefrom, or conversely to input an additional light signal into the fiber. Alternately, in a telecommunications system, the presence of an unwanted microbend coupler to tap information from the fiber can be detected by monitoring signal attenuation.
The concept of optical fiber microbending has also been proposed as a transducer mechanism for sensing and quantifying pressure acting upon a physical structure, such as a diaphragm or pressure plate. In this type of application, a so-called microbend transducer is mounted on the structure for movement therewith in response to pressure to induce microbending of an optical fiber. The microbending causes a detectable attenuation of a light signal passing through the fiber, wherein the degree of attenuation is indicative of the magnitude of pressure. For a discussion of a microbend pressure transducer, see Fields et al.: "Fiber Optic Pressure Sensor", J. Acoust. Soc. Am., March, 1980, pages 816-818.
In some environments, it is necessary or desirable to monitor the location and magnitude of selected loads acting upon a physical structure, typically by monitoring a plurality of force transducers mounted along the length of the structure. For example, it is highly desirable to locate and quantify localized stress to which an oil or gas pipeline is subjected, primarily as a result of variations in weather and ground elevation, so that remedial measures can be taken prior to breakage of the pipeline. This problem of pipeline stress is particularly troublesome when the pipeline travels through expansive regions of wilderness or wasteland, such as the so-called Alaskan pipeline which extends for hundreds of miles over relatively unstable tundra. However, for this type of application, conventional transducers such as piezoelectric transducers have not been used because of the hostile conditions to which the transducers would be exposed together with the difficulty in remote monitoring of a large number of the transducers from a single monitoring station. Moreover, while transducers of the microbend type have been proposed for detecting pressure acting upon a structure, no practical and effective system has been proposed or demonstrated for adapting a microbend transducer to a pipeline stress application or for monitoring a large number of microbend transducers from a single monitoring station.
The present invention overcomes the problems encountered in the prior art by providing a practical and effective optical fiber and microbend transducer system for detecting the location and magnitude of forces, such as stress forces, acting along the length of a structure, such as an oil or gas pipeline, wherein a large number of force-responsive microbend transducers are capable of being monitored simultaneously from a single remote monitoring station.